There is great concern among world health organizations and certain regulatory agencies regarding the quality levels of protective barrier materials or products, for example, condoms and gloves. This concern is due to the urgent need to protect users of the barrier materials against the transmission of fluid transmitted diseases, for example, the AIDS virus, having a diameter of about 0.1 micron (.mu.), and the herpes virus, having a diameter of about 0.12 to about 0.15.mu.. The greatest concerns are for freedom from bursting and/or tearing of the barrier materials during use and freedom from the presence of holes, for example, pinholes which are large enough to permit transmission of viruses, which may allow the transmission of such diseases. Accordingly, the present invention has been developed for the testing of the integrity of such protective barrier materials. In particular, the present invention has been developed for the non-destructive testing of protective barrier materials for holes having a diameter less than a micron.
Manufacturing high quality condoms, gloves and other barrier material products requires a well-trained technical staff and special production equipment. Most large condom manufacturing plants are highly automated and have been custom designed by the individual condom manufacturer.
Vulcanization processes for rubber were developed in the late nineteenth century. This enabled the substitution of rubber for animal membranes in the manufacture of condoms. Dissolved rubber was initially used as the raw material for condoms. However, since the stabilization of dissolved rubber particles was poor, a high percentage of defective condoms were initially produced. In the 1930's, production of condoms from liquid natural rubber latex revolutionized the rubber and condom manufacturing industries.
During the current manufacture of condoms, concentrated and stabilized latex is placed in dipping tanks into which glass or metal condom-shaped molds, or mandrels, are dipped by means of a continuous conveyor system. The molds are then carried through a tank containing cooled latex and are placed into drying ovens. The latex-covered molds are then dipped into a second tank of latex. A brush rolls a ring on the open end of the condom and the molds are then conveyed into ovens for further drying and vulcanization. The condoms are then stripped from the molds by water jets or brushes and the condoms are tested for integrity.
While rubber condoms or gloves are generally produced in various sizes using molds, testing is carried out on mandrels also of different sizes. Rubber membranes can be produced as thin as 0.02 millimeter (mm). However, most rubber membranes are produced having a thickness of about 0.04 to about 0.07 mm. The thinner the membrane, the greater the sensitivity for the user. However, thicker membranes are stronger and less likely to have holes, thereby having higher and therefore more acceptable quality levels.
Currently, there are two primary types of testing procedures utilized for testing condoms: hydraulic and electrical procedures. The hydraulic and electrical tests are both influenced by the surface properties of the latex (the most common condom material) and the surface energy of the liquid which is used in the tests. The wettability of any pore surface of a condom is the primary determinant of the formation of a fluid pathway across a condom. Thus, the testing of condoms for integrity from defects, for example, holes, depends on the state of the interface.
According to the American Society for Testing and Materials (ASTM D3492-83) hydraulic test, the condom is filled with about 300 milliliters (mL) of water and is then visually inspected for water leakage. The FDA uses a modified version of the ASTM hydraulic test in which the condom is filled with 300 mL of water and the open end is then closed. The water-filled condom is then rolled on a water-absorbent material, which is subsequently visually inspected for spots of water.
Condoms are also tested individually using an electrical screening process. Unfilled latex rubber, such as the latex rubber used in condoms, is a very poor electrical conductor so that electrical current generally does not pass through an unflawed or defect-free condom. In the electrical screening process, condoms are rolled by hand onto tube-shaped electrodes or mandrels and are then passed through an electrolyte solution (wet system) or across a fine mesh screen (dry system) during the application of an electrical potential between the electrode and the electrolyte or screen. A condom is rejected if current passes through it and/or the current increases.
As indicated, the electrical screening tests fall into two categories: dry and wet electrical tests. The dry electrical test comprises a high voltage dielectric breakdown test, wherein a condom is mounted on a metal mandrel (the inside electrode) and is subjected to a high voltage from an outside electrode. The dry electrical test is actually a semi-destructive test, since failed condoms are generally irreversibly damaged.
There are several different varieties of tests which may be considered wet electrical tests. However, all of the wet electrical tests directly or indirectly measure the resistance across a hole in the condom. The resistance is measured directly by measuring the direct current (d.c.) resistance of a condom-covered mandrel to the metal tank containing an electrolyte, and indirectly, by measuring the voltage remaining on a charged condom-covered mandrel after an elapsed time period.
D.C. resistance measurement is the industry standard used today for hole and potential leakage testing. However, this method is subject to secondary effects, namely, large d.c. polarization and interfacial resistance effects at the membrane holes. Moreover, the magnitude of the secondary effects increases as the hole becomes smaller, resulting in a greater uncertainty in the measurement. Thus, smaller holes, for example, pinholes with a diameter in the micron or sub-micron range, have a considerable increase in their effective d.c. resistance. This makes it more difficult to detect such pinholes using such a d.c. resistance measurement method. In particular, the reliability of d.c. resistance measurement for defect detection at the level required for protection against the AIDS virus and other sub-micron-sized viruses is now being questioned by the regulatory agencies and other health organizations.
In addition, resistance tests have sometimes been used as laboratory tests for lot assessment. Laboratory measurements on commercially available condoms have demonstrated that in a given production lot, there may be some condoms with small and large holes, while in another given lot, there may not be any defective condoms. Thus, reliable testing of each condom is needed.
With the current electrical testing methods, the magnitude of the impedance of the barrier material is measured. However, these tests include the reactive component of that impedance which is due to the capacitance of the condom. Furthermore, in most instances, manufacturers use simple tap water as the electrolyte in their electrical tests. Tap water is quite resistive and the detection limit for holes in the barrier material increases to about 60.mu. to about 80.mu.. The present invention is directed to overcoming the shortcomings of prior art testing techniques for protective barrier materials. The testing procedure of the present invention makes use of the phenomena that surface tension lowering of the contacting aqueous phase facilitates the filling of small pores and the transport of fluid or particles in the fluid by wetting of the latex barrier.